1. Field of the Invention
The present invention relates generally to boosted internal combustion engines and air compressors used therein. More particularly, the invention relates to variable displacement internal combustion engines and air compressors used therein adapted to operate over a wide range of airflow rates.
2. Background of the Invention
As is known in the art, for conventional boosted internal combustion engines, such as supercharged or turbocharged engines, an air compressor pressurizes air in the intake of the engine. By increasing density in the intake, the engine inducts and combusts a greater amount of air and fuel, thereby increasing the power output of the engine. One such compressor is a centrifugal compressor having: an impeller rotatable about an axial shaft for forcing air entering along the axis outwardly toward the outer circumferential region of the compressor, an outlet disposed along a portion of the circumferential portion, and a diffuser disposed in the outlet for converting the kinetic energy of the air forced toward the circumferential portion into air pressure as the air passes through the outlet. Diffuser is a term of art applied to centrifugal compressors. Air at high velocity enters the diffuser and is slowed down in the diffuser with a consequent increase in pressure. Typically, the diffuser contains stator blades (alternatively called guide vanes) to provide efficient energy conversion. Alternatively, the diffuser can be without blades or vaneless.
As is also known in the art, the operating range of the air compressor is limited by choking at high airflow rates and surging at low airflow rate. Surge is an unstable operating condition, which is to be avoided. In the stable operating region of a compressor operating map, when outlet air flow is restricted, the pressure rises to counteract the restriction. But, in the unstable region of the operating map, a further restriction causes the pressure of the compressor to fall. Higher pressure air in the delivery pipe surges back through the compressor. But, with this pressure relief, the compressor responds by rebuilding pressure, causing flow in the forward direction. The constant flow reversals lead to this surging condition. It is typical to match the engine demands to the compressor to avoid choking or surging. However, this presents a challenge, particularly in gasoline engines, which are typically throttled, when the ratio between the highest and lowest airflow rates is great.
As is also known in the art, one type of internal combustion engine is a variable displacement engine (VDE) in which a portion of the cylinders are deactivated during low power conditions. When boosting is used with such VDE, it has been proposed to use two or more air compressors separately coupled to active or inactive cylinders. In such an arrangement, the compressor coupled to inactive cylinders is deactivated. Such a solution, however, is costly and complex.
U.S. Pat. No. 5,310,309 discloses a centrifugal compressor, which is designed for avoiding surging at low airflow rates. Stator blades are arranged in the diffuser of the compressor. The stator blades have leading edges inclined in the downstream direction while extending away from a side plate toward a core plate. Auxiliary blades are arranged at positions inward of the stator blades. The auxiliary blades shift the surge limit to lower airflows. However, these blades do not shift the surge limit to the extent needed for a high reduction in airflow, for example when using the compressor to charge a VDE, where the airflow is reduced drastically when some cylinders are deactivated. Furthermore, an additional manufacturing step is used to provide a compressor with auxiliary blades.
In accordance with the invention, a compressor is provided having a compressor housing defining an air inlet and a plurality of air outlets adapted to be fed air from the air inlet. A rotatable shaft with a plurality of impeller blades coupled to said rotatable shaft is disposed within the housing. A plurality of diffusers is provided, each one of the diffusers being coupled to one of the air outlets.
With such an arrangement, a compressor is provided having a common housing with a plurality of air outlets.
Further, with such an arrangement, airflow from one of the air outlets can be changed relative to the airflow from the other one of the air outlets. In one embodiment, the airflow from one outlet can be stopped so that air flows through the other outlets only. In the case of a two air outlet compressor, the velocity of the air flowing through the open outlets and the corresponding diffuser therein is roughly twice that compared to having one outlet due to the reduction in cross-section. Consequently, the compressor is less likely to access the surging condition.
An advantage of the invention is that the surge limit of a compressor with the two air outlets is shifted to lower airflow. This is useful under the following exemplary operating conditions of the engine: low engine speed, at closed throttle, or when operating with some of the cylinders deactivated in a VDE engine.
Further, with such an arrangement, the compressor unit has, in effect, a plurality of compressors housed in one package.
In one embodiment, at least one partition wall separates a pair of the plurality of diffusers.
In one embodiment, each of the impeller blades has a slot extending radially inward, and the partition wall between each pair of diffusers extends radially into the slots of the impeller blades.
In one embodiment, a separator is coupled to the impeller blades.
In one embodiment, the partition wall is positioned substantially perpendicular with respect to an axis of rotation of the shaft.
In accordance with another feature of the invention, a variable displacement engine is provided. The engine includes a plurality of sets of combustion cylinders. A compressor is provided having a rotatable shaft; a compressor housing defining an air inlet and a plurality of air outlets; a rotatable shaft disposed within the housing; and a plurality of impeller blades coupled to the rotatable shaft and disposed within the housing; and a plurality of circumferential diffusers disposed within the housing, each one of the diffusers being coupled to a corresponding one of said air outlets which is further coupled to a corresponding one of said set of combustion cylinders. At least one of said sets of combustion cylinders is deactivated, thereby causing flow in said diffusers corresponding to said deactivated combustion cylinders to substantially stop.
In a further embodiment of the invention, a partition wall, or septum, is disposed between the two air diffusers. The partition wall divides the airflow so that the air is divided as it comes off the impeller blades. During low airflows, efficiency of the compressor is improved when one of the outlets is closed. In one of the diffusers, the air passages are smaller so that the streaming air is of a velocity within the diffuser not within the surge limit.
In one embodiment, each impeller blade has a slot extending radially inward and the partition wall of the compressor housing between the diffusers of such air outlets extends radially into the slots of the impeller blades, forming two sub-compressors. In this way, the flow is substantially divided within the impeller section.
In a further embodiment, the compressor wheel has a separator coupled to the impeller blades. The separator separates the airflow near the compressor shaft to shift surging of the compressor to lower airflows. In this embodiment, the separator causes the flow to divide at the upstream portion of the impeller blades.